Contraction of heart muscle results from the cyclic interaction of myosin with actin under the regulation of sarcomeric thin and thick filament proteins. Based on results from skinned fibers and cells in culture it has been proposed that phosphorylation of the regulatory light chain (RLC) of sarcomeric myosin plays a role in enhancing ventricular contraction while phosphorylation of either sarcomeric or cytoplasmic myosin-IIB promotes sarcomere assembly. Functional changes in the expression and activity of Ca2+/calmodulin- dependent myosin light chain kinase (MLCK) may lead to cardiomyopathy. Although cardiac myocytes contain smooth (sm) muscle MLCK, it is not clear that this is the only kinase responsible for RLC phosphorylations. We propose to identify the kinase that phosphorylates the respective RLCs in addition to physiological roles for RLC phosphorylations in cardiac contraction and sarcomere assembly. Specific Aim 1. Determine effects of overexpression and knockout of smMLCK in ventricular and atrial myocytes. Transgenic mice will be made with MLCK expressed specifically in cardiac myocytes. The gene for smooth muscle MLCK will be ablated by crossing mice with the floxed smooth muscle MLCK gene and mice expressing Cre specifically in cardiac myocytes. Other kinases that phosphorylate RLC will be identified. RLC phosphorylations will be measured in cardiac tissues as well as isolated myocytes. Specific Aim 2. Determine if functional consequences are associated with changes in expression of MLCK isoforms. Anatomical properties of hearts from transgenic and knockout mice will be assessed. Functional properties including contractile indices will be measured in perfused hearts and in vivo by invasive and noninvasive measurements. Responsiveness to stresses that lead to cardiac hypertrophy will also be evaluated. Specific Aim 3. Determine if sarcomere assembly is affected by overexpression or knockout of smMLCK. Sarcomere formation in response to hypertrophic agents will be measured in myocytes from newborn transgenic or knockout mice. Morphometric analyses will be combined with RLC phosphorylation measurements. Development of myofibrillar structure will be examined in hearts from embryonic knockout mice. Results from these studies will provide novel biochemical insights into the physiological regulation and functional importance of cardiac myosin light chain phosphorylation.